Thermal ballast

ABSTRACT

A thermal ballast temporarily stores heat generated during a heat generating device&#39;s operation thereby extending the device&#39;s operating time during an interval of adverse thermal operating condition. The thermal ballast ( 10 ) includes:
         a. a highly thermally conductive layer ( 42 ) that is adapted for being thermally coupled to the heat generating device; and   b. a high heat capacity layer ( 44 ) that is laminated to the high thermal conductivity layer ( 42 ).
 
The high heat capacity layer ( 44 ) may be formed from a solid piece of a suitably chosen material. Alternatively, the high heat capacity layer ( 44 ) may be formed from a piece of a suitably chosen material having a cavity that is filled with a material ( 46 ) which changes phase at a temperature below the heat generating device&#39;s maximum operating temperature. Disclosed are various different ways for infusing the material ( 46 ) which changes phase at a temperature below the heat generating device&#39;s maximum operating temperature into the high heat capacity layer ( 44 ) and retaining the material ( 46 ) therein.

CROSS REFERENCE(S) TO RELATED APPLICATION(S)

This patent application is a continuation-in-part of U.S. patentapplication Ser. No. 15/999,700 that was filed with the United StatesPatent and Trademark Office (“USPTO”) on Aug. 20, 2018, as acontinuation-in-part of U.S. patent application Ser. No. 15/635,143 thatwas filed with the USPTO on Jun. 27, 2017, claiming the benefit of U.S.Provisional Patent Application No. 62/355,181 that was filed with theUSPTO on Jun. 27, 2016.

BACKGROUND Technical Field

The present disclosure relates generally to liquid crystal displays(“LCDs”), and more particularly to enhancing an LCD's perceptibilitywhen in sunlight.

Background Art

Popularity of LCDs have imposed increased demands on their performanceparticularly when included in portable electronic products such as smartphones, personal digital assistants (“PDAs”), notebook computers(“notebook PCs”), tablet computers (“tablet PCs”) and so forth. However,LCDs in such portable electronic devices must provide an image that isvisible both indoors and outdoors, i.e. in the presence of sunlight.Typically, moving a portable electronic product into strong sunlightwashes out the image on the display screen. The main reason that imageson LCDs are difficult to view outdoors is that sunlight overpowers abacklight emitting light that passes through the LCD while sunlightreflects from the LCD screen. Presently, increasing backlight brightnessis commonly used for improving LCDs visibility in glaring light.However, increasing a LCDs visibility in glaring light by increasingbacklight brightness is disadvantageous because:

1. a brighter backlight increases a device's electrical powerconsumption;

2. increased power consumption accelerates the drain of the systembattery; and

3. increased power consumption requires greater thermal dissipation orthe LCD will likely overheat.

Historically, and particularly for LCD backlighting units (“BLUs”) thatincludes a light pipe, BLUs usually operate as a fixed brightness lightsource at a constant maximum brightness. From a practical perspective,operating BLUs at a constant maximum brightness was unavoidable untilLED-based BLUs began replacing cold cathode fluorescent (“CCFL”) basedBLUs. Using light emitting diodes (“LEDs”) in a LCD's BLU is relativelyrecent. Beginning around 2008 LED BLUs began rapidly replacing CCFLillumination in LCD BLUs.

FIGS. 1-3 depict a typical commercially designed LCD BLU, identified bythe general reference number (18), juxtaposed with a LCD glass panel(04). For purposes of the present description, the relevant componentsof the BLU (18) include LEDs (01) arranged in an approximately linearLED array (21) stationed in an edge-lit configuration with respect to alight pipe (03). Light emitted from the LEDs (01) enters an input edge(02) of the light pipe (03) which transforms the light into anapproximately uniformly lit rectangle that back illuminates the LCDglass panel (04) producing a screen brightness B(0). A typicalcommercial LCD module includes other mechanical, electrical and opticalcomponents in addition to the BLU (18) that are irrelevant to thisdescription and, for clarity, have been omitted from the drawings.

Drive electrical current supplied to LEDs (01) is largely limited bythermal dissipation of the lighting system design and by self-heating ofLEDs semiconductor junction which is the principle source of heatgeneration within the BLU (18). The allowable operating electricalcurrent flowing through LEDs (01) is governed far more significantly bythese thermal considerations than a CCFL's operating electrical current.However, an LED (01) may be safely driven at a substantially higherelectrical current than its normal continuous operating drive electricalcurrent for short periods of time. The length of such periods depends onthe effectiveness of the thermal design of the BLU (18) and theintrinsic thermal conductivity of the LED light source carrier package.Under appropriate circumstances, it is possible to advantageouslyexploit this fact to temporarily boost the light output of an LED BLU(18) by as much as 2-6 times or more above its continuous operatingbrightness.

United States Patent Application Publication no. 2012/0188481 A1entitled “LCD Apparatus” discloses a LCD display apparatus adapted foruse outdoors. The disclosed LCD display apparatus includes a pair of LCDmodules arranged back to back so each LCD is viewable from one side ofthe LCD display apparatus. Each LCD module includes a LCD and itsassociated back panel that includes a backlight module. The LCD modules'back panels, which face each other, are spaced apart to establish aventilation channel between the back panels. A set of fans located atone end of the ventilation channel blow air longitudinally past the backpanels to remove heat generated in the backlight modules and otherelectronic circuits mounted thereon. A backlight controller respondingto an ambient light sensor dynamically controls operation of thebacklighting module so LCD backlighting responds to ambient lighting formaintaining a balance between sufficient image brightness, energypreservation and operating life of the LCD panel.

LEDs (01) are commonly known as current devices. They are best poweredfrom a circuit that regulates their input current by carefullymonitoring the LED drive current via an active feedback loop. Such a LEDdriver integrated circuit (“IC”) chip continuously adjusts voltagesupplied to the LEDs (01) to maintain a specified electrical currentthrough the LEDs (01). While many such LED driver ICs are commerciallyavailable, two examples thereof are a LTC3783 PWM LED Driver and Boost,Flyback and SEPIC Controller made by Linear Technology Corporation 1630McCarthy Boulevard, Milpitas, Calif., and a HV9912 Switch-mode LEDDriver IC With High Current Accuracy and Hiccup Mode Protection made bySupertex, Inc. 1235 Bordeaux Drive, Sunnyvale, Calif.

Output to the LEDs (01) can be user modified from an appropriate inputusing either pulse width modulation (PWM) or direct control of thedriver circuit DC output current. Most LED driver chips have inputs forthis kind of control which, of course, can be done by some kind ofmanual input or automatically in response to an external command as froma microprocessor or other data processing device. These in turn can havevarious ambient sensing inputs such as thermal and optical sensorsgiving the system some amount of intelligent decision making ability. Inany case, given the very fast response time of typical LEDs (01), it ispossible to give a user the ability to rapidly adjust the BLUbrightness.

BRIEF SUMMARY

An object of the present disclosure is to provide a better BLU (18) forviewing a LCD in bright sunlight.

Another object of the present invention is to slow the rate oftemperature increase in the LEDs (01) by advantageously exploiting aphase transition occurring in a suitably selected Phase Change Material(“PCM”).

Briefly, the present disclosure includes a method for operating BLU (18)that includes a LED array illumination source. Supplying an electricalcurrent continuously to LEDs included in the LED array causes the LEDarray to emit illumination that passes through a LCD. The methodincludes a step of increasing for a brief period of time electricalcurrent supplied to the LEDs.

During the brief period of time while the increased electrical currentflows through the LEDs, the illumination passing through the LCDincreases significantly thereby permitting improved viewing the LCD whenin bright sunlight. The present disclosure differs from conventional LEDBLU in that the BLU (18) is energized for a short interval of time so asto emit a significantly higher brightness than the LED BLU (18) emitsduring continuous operation at maximum brightness.

Advantages of the present disclosure for momentarily increasingelectrical current flowing through LEDs (01) included in the BLU (18)beyond that normally flowing continuously through the LEDs (01) include:

1. reducing cost of a display system capable of such operation;

2. reducing overall electrical power consumed by the BLU (18)significantly thereby either:

-   -   a. extending battery life; and/or    -   b. allowing use of a smaller, lighter battery; and

3. reducing both the weight and size of a display system capable of suchoperation.

A preferred embodiment BLU (18) that operates in accordance with thepresent disclosure includes more LEDs (01) in the LED array than areincluded in a conventional LED array. Including more LEDs in the LEDarray provides two additional benefits for the end user.

1. When operating the BLU (18) continuously at a brightness typical ofmost commercially designed displays, a lower average LED drive currentsignificantly increases:

-   -   a. LED light generating efficiency; and    -   b. correspondingly decreases electrical power consumed by the        BLU (18) and heat produced thereby extending operating time for        a battery energized device.

2. Decreasing per-LED power consumption and heat generation improves theoperating life of the BLU (18) and reliability.

A second object of the present disclosure is to provide a means forextending the safe operating time of the BLU (18) under high thermalload by incorporating a planarized thermal ballast that is in thermalcontact with the LEDs (05, 21) of the BLU (18). This preferredembodiment BLU (18) provides a means for extending the total duration ofbrightness boost by slowing down the rate of temperature rise caused bythe increased power being supplied to the BLU (18).

These and other features, objects and advantages will be understood byand/or apparent to those of ordinary skill in the art from the followingdetailed description of the preferred embodiment as illustrated in thevarious drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-sectional elevational view of a typicalcommercial LCD BLU (18) depicting LEDs of a LED array and of a lightpipe for projecting illumination through a LCD;

FIG. 2 is a cross-sectional elevational view of a typical commercial LCDBLU (18) depicting LEDs of the LED (21) array and of the entire lightfor pipe projecting illumination through the LCD;

FIG. 3 is a partial perspective view of a portion of a typicalcommercial LCD BLU (18) depicting LEDs mounted of the LED array (21) andthe light pipe for projecting illumination through the LCD;

FIG. 4 is an elevational view comparing a density of LEDs of the LEDarray (21) included in a typical commercial LCD BLU (18) with a greaterdensity of LEDs of a LED array in accordance with a preferred embodimentBLU (18) of the present disclosure;

FIG. 5 is a cross-sectional elevational view of a LCD BLU (18) inaccordance with the present disclosure depicting LEDs of the LED arraydepicted in FIG. 4 and of the entire light pipe for projectingillumination through the LCD;

FIG. 6 is a partial perspective view of a portion of the LCD BLU (18) inaccordance with the present disclosure's preferred embodiment depictingLEDs of the LED array shown in FIG. 4 with the greater density LEDsmounted thereon for emitting light into the BLU's light pipe;

FIG. 7 is a plan view illustrating a portable LCD tablet device thatincludes a BLU (18) in accordance with the present disclosure having abutton thereon that when depressed causes the BLU (18) to temporarilyincrease illumination projected from the BLU (18) through the LCD;

FIG. 8 is an exploded perspective view of the portable tablet devicedepicted in FIG. 7 having a BLU (18) in accordance with the presentdisclosure;

FIG. 9 is a block diagram of a LCD system in accordance with the presentdisclosure;

FIG. 10 is a flowchart of a LCD system in accordance with the presentdisclosure;

FIG. 11 is a side elevational view of a PCM based, planarized, thermalballast juxtaposed with a LED heat source in accordance with the presentdisclosure;

FIG. 12 is a perspective view of the PCM based, planarized, thermalballast depicted in FIG. 11;

FIG. 13 is a partially sectioned perspective view depicting a pouchfilled with a PCM that, in accordance with the present disclosure, ispreferably included in the thermal ballast depicted in FIGS. 11 and 12;

FIG. 14 is a side elevational view of a finned heat sink that, similarto the thermal ballast depicted in FIGS. 11 and 12, includes a PCM;

FIG. 15 is a perspective view of the finned heat sink depicted in FIG.14;

FIG. 16 is a perspective view of an alternative embodiment of the finnedheat sink depicted in FIGS. 14 and 15 that further including thermalvias for increasing heat conduction into the PCM;

FIG. 17 is a graph presenting results of calculated temperature of LEDs(01) versus time for:

-   -   1. four different planarized thermal ballast configurations with        each configuration being integrated with a PCs tablet's 10.4        inch LCD display wherein the display's LED array (05, 21) is        driven with 8 watts of input power: and    -   2. one configuration wherein the LED array (05, 21) is driven        with 4 of input power;

FIG. 18 is a graph, similar to the graph of FIG. 17, presenting resultsof calculated temperature of LEDs (01) versus time differing primarilyfrom the graph of FIG. 17 in that the display's LED array (05, 21) isdriven with 12 watts of input power; and

FIG. 19 is a graph, similar to the graphs of FIGS. 17 and 18, presentingresults of calculated temperature of LEDs (01) versus time differingprimarily from the graphs of FIGS. 17 and 18 in that the display's LEDarray (05, 21) is driven with 16 watts of input power.

DETAILED DESCRIPTION

The present disclosure advantageously uses, for enhancing daylightvisibility of a LCD, the ability to substantially increase, for shortperiods of time, electrical current flowing through LEDs (01) includedin the BLU (18) above the normal continuous operating electricalcurrent. During the short periods of time, electrical current suppliedto the LEDs (01) of the BLU (18) increases to an amount above thatsupplied for continuous operation. Because of the widespreadavailability and use of low cost logic and data processing devices,there are numerous ways to configure an electronic system to accomplishthe feedback and control functions described in this disclosure. Thiswould be readily apparent to those skilled in the art. However, thefunctional elements described herein will detail the basic requirements.A BLU brightness control (22) in accordance with the present disclosure,depicted in FIGS. 9 and 10, permits a user to rapidly adjust the rootmean square electrical current supplied to LEDs (01) or the BLU (18).But the relevant logic device prevents the user from re-increasing theelectrical current too frequently so increased heat generated in theLEDs (01) can be adequately dissipated thereby avoiding LED damage.

As with most electronic components, typical engineering design practicesets LED backlight maximum drive current below the absolute maximumcontinuous drive operating current specification for a particular LED(01). However, in a BLU (18) operated in accordance with the presentdisclosure, the thermal mass of the LEDs (01) plus that of an associatedheat sinking planarized thermal ballast (10) and dissipation componentsallows significantly increasing electrical current flowing through theLEDs (01) so long as the increase in electrical current is keptrelatively short in duration, for example 15 seconds.

FIG. 4 illustrates a preferred embodiment LED array (05) particularlyadapted for operation in accordance with the present disclosure. FIG. 4graphically depicts a difference in LED density between the standard LEDarray (21) and a preferred embodiment LED array (05). Consequently, theLED array (05) includes considerably more LEDs (01) than a typical LEDarray (21) depicted in FIGS. 1-3. Because the LED array (05) has ahigher density of LEDs (01) than the standard LED array (21), thepreferred embodiment LED array (05):

1. produces significantly more light than LED array (21) when the sameamount of electrical current flows through the array of LEDs (01); or

2. alternatively produces the same amount of light for a lesser amountof electrical current flowing through each LED (01) included in the LEDarray (05).

FIGS. 5 and 6 illustrate a BLU (18) that includes the LED array (05)depictions that respectively correspond to FIGS. 2's and 3's drawings.Without damaging the LEDs (01), a much larger total electrical currentcan flow through LEDs (01) of the LED array (05) than can flow throughthe conventional LED array (21). This larger total current flowingthrough LEDs (01) of the LED array (05) correspondingly increases screenbrightness B(nos) indicated by arrows in FIG. 5. The increased screenbrightness B(nos) resulting from light emitted from the LED array (05)that passes through the LCD glass panel (04) depicted in FIG. 5typically ranges between one and one-quarter (1.25) to six (6) times thecontinuous screen brightness B(0) of light passing through the LCD glasspanel (04) for the conventional BLU (18) depicted in FIGS. 1-3.

FIG. 7 depicts an exemplary high brightness portable tablet (06) whichincludes a BLU (18) in accordance with the present disclosure. Thetablet (06) includes a NOS button (07) on the front thereof. To increasethe brightness of an image appearing on the tablet (06) a user pressesthe NOS button (07). Specific details of how much the brightnessincreases and for how long are controlled by parameters that can eitherbe preset by a manufacturer of the tablet (06) or, programmed by a userof the tablet (06).

The exploded perspective view of FIG. 8 illustrates various componentsof the exemplary high brightness tablet (06) that is adapted foroperating in accordance with the present disclosure. Typically, the LCDmodule (08) includes the LCD glass panel (04), LCD drive electronics notseparately depicted in any of the FIGs., a frame and related mechanicalmounting means. A BLU (18) that includes the LED array (05) may includeone or more thermal sensors (16) for monitoring temperature of the BLU(18). The high brightness tablet (06) includes a LED driver (09) that iscapable of supplying sufficient electrical current both:

-   -   1. for continuous operation of the LED array (05) or        alternatively the standard density LED array (21); and    -   2. for increasing an image's brightness when a user presses the        NOS button (07).        The LED driver (09) may also provide all ancillary feedback and        control functions required for operating the BLU (18) in        accordance with the present disclosure. Alternatively, such        functions may reside in other components of the high brightness        tablet (06).

The optional pair of thermal sensors (16) that may be included in theportable tablet (06) preferably locates one thermal sensors (16)adjacent to the middle of the LED array (05, 21), and the other thermalsensor (16) preferably adjacent to the middle of the thermal ballast(10) at an end thereof that is furthest from the LED array (05, 21). Ifthe BLU (18) includes a thermal sensor (16) depicted in FIGS. 8-10 whichresponds to the temperature of the BLU (18), this provides an electricalsignal to the data processing system (“DPS”) (13) which then determinesoperating parameters that prevent the brightness increase from damagingthe LEDs (01) included in the LED array (05, 21).

Also illustrated in FIG. 8 and included in the high brightness tablet(06) is a heat conducting, spreading and temporary storage thermalballast (10) typically made from graphite, aluminum or copper.Alternatively, the thermal ballast (10) could also be a laminatecombining high thermal mass materials and high thermal conductionmaterials such as graphite, aluminum or copper respectively.

One end of the thermal ballast (10) is juxtaposed with the LED array(05) which in the illustration of FIG. 8 is located inside the LCDmodule (08) horizontally at the bottom thereof. The term “thermallycoupled” means that the thermal ballast (10) is mechanically affixedadjacent to or in very near proximity to the source or sources of heatproduced by the LEDs (01) such that there exists a relatively lowthermal resistance heat path from the LEDs (01) into the thermal ballast(10). The thermal ballast (10) conducts excess heat generated by the LEDarray (05) away from its LEDs (01), most importantly during intervals inwhich an image's brightness is being increased. The thermal ballast (10)temporarily stores heat produced by the LED array (05) while an image'sbrightness is being increased. In essence, this arrangement reduces therate at which the temperature of the LEDs (01) increases while animage's brightness is being increased. The amount of thermal massprovided by the thermal ballast (10) together with the amount of addedpower consumed by the LED array (05) correlates directly with how longthe high brightness tablet (06) may present a brightened image.

FIG. 9 is a block diagram of a display system in accordance with thepresent disclosure. The block diagram illustrates the preferredembodiments in the present disclosure which include a planarized thermalballast (10) and one or more thermal sensors (16) for monitoring thetemperature of the planarized thermal ballast (10). Both a boostrequest, that a user may present by pressing the NOS button (07), and asystem set-up input (31), appear in the FIG. 9 block diagram. An arrow(23) in FIG. 9 indicates transmission of the boost request to the DPS(13), and an arrow (24) in FIG. 9 indicates transmission of system setupinformation to the DPS (13). The DPS (13) illustrated in FIG. 9 includesa clock icon to indicate that lacking either the thermal sensors (16) orplanarized thermal ballast (10), the DPS (13) is capable of determiningif a brightness increase is still possible based solely upon timingconsiderations, as explained in the next paragraph.

FIG. 10 illustrates a functional flowchart of feedback and controlfunctions for controlling brightness increase(s) by the user inaccordance with the present disclosure. Starting from a brightness boostrequest that a user may present by pressing the NOS button (07), asignal is transmitted to decision block (35) as indicated by an arrow(23). The DPS (13) in performing decision block (35) determines if it issafe to allow the brightness increase request. This is minimallydetermined by the BLU's thermal design and the user's amortizedcumulative brightness increase request(s). If a BLU-associated thermalsensor (16) is included in the system design, then as indicated by anarrow (25) in FIGS. 9 and 10, the sensor (16) supplies accurate realtime data for use by the DPS (13) in analyzing the user request.Otherwise, the DPS (13) must rely on embedded designer-programmeda-priori knowledge of the un-boosted BLU power consumption obtainedduring product design and testing. In this case, the designer(s) mustrely on timing and experimentally measured heat dissipationcharacteristics of the system to appropriately program the response ofthe DPS (13) to brightness boost requests.

In FIG. 10, an arrow (26) indicates transmission of a control signalfrom the decision block (35) to the BLU brightness control (22) forpermitting a brightness increase by the LED array (05, 21). Conversely,an arrow (27) in FIG. 10 indicates transmission of a control signal fromthe decision block (35) to the BLU brightness control (22) formaintaining the continuous brightness setting of the LED array (05, 21),i.e. for blocking a brightness increase by the LED array (05, 21).

In both FIGS. 9 and 10, an arrow (28) depicts transmission of a signalfrom the BLU brightness control (22) to the LED driver (09) forcontrolling the brightness of the LED array (05, 21). An arrow (29) inFIG. 9 indicates transmission of electrical current from the LED driver(09) to the high density LED array (05) or alternatively the standarddensity LED array (21). Arrows (30) and (32), respectively appearingboth in FIGS. 9 and 10, indicate transmission of heat from the LED array(05, 21) via the thermal ballast (10) to the thermal sensors (16).

As illustrated in FIG. 8, the exemplary high brightness tablet (06)includes a front (14) and rear (15) of a case that enclose othercomponents included therein. While not required for practicing thepresent disclosure, as illustrated in FIG. 8 the exemplary highbrightness tablet (06) will generally include a lithium ion battery(11), a touch panel (12) and a DPS (13) such as an ARM-based singleboard computer. The DPS (13), different types of which are widelyavailable, will typically include a video/graphics controller tointerface with and drive the LCD module (08) as well as both a USB portand a HDMI port, that are not separately illustrated in FIG. 8. If theLED driver (09) of the BLU (18) were to omit feedback and controlfunctions, alternatively such functions could be included in firmwareexecuted by the DPS (13). The touch panel (12) requires a controllerwhich may or may not be integrated into the DPS (13) and/or the LEDdriver (09). The DPS (13) may also include a Wi-Fi transceiver to allowthe tablet (06) to video link wirelessly with a video camera such as isfrequently included in a smart phone, tablet or any other appropriatelycapable mobile computing platform, and/or telemetry received from adrone or drone controller or any other remotely controlled system.

Temporarily boosting brightness helps applications such as commercialdrone controllers which are most often operated outdoors in daylightwhere the sun can easily wash out the drone camera image and/ortelemetry feeds appearing on the pilot's video monitor. For example, adrone's operator who is outdoors in daylight relies to some degree onthe drone's video camera and telemetry data to control the drone'sflight and operation. However, in such an environment viewing a videomonitor can become difficult because of ambient reflections and sunlightfalling on the front (14) of the display screen. When the drone'soperator becomes aware that the drone is entering a critical part of itsflight, the operator wants to ensure being able to clearly seeeverything on the LCD. Therefore, when such an event occurs the drone'soperator can, in accordance with the present disclosure, initiate aperiod of increased screen brightness B(nos) having a preselectedduration, e.g. 10 seconds.

Since most flight control systems for commercial drones are batteryoperated, if the BLU (18) operated continuously at the increased screenbrightness B(nos), the flight control battery would discharge much morerapidly. However, in accordance with the present disclosure a drone'soperator can view the drone's video and/or telemetry feeds under allambient conditions without increasing the flight control's weight and/orwhile avoiding significant compromise of the flight control's batterylife. As is readily apparent, any outdoor daylight application of an LCDdisplay could advantageously operate in accordance with the presentdisclosure.

A possibility exists that one could simply increase the drive currentprovided to a conventionally designed LED illuminator array (21), as aretypically found in consumer grade electronic devices, to boost thescreen brightness B(nos). But such operation of a conventional BLU (18)has the disadvantages of:

1. significantly shortening the operating life of the BLU (18);

2. decreasing the reliability of the LCD module (08); and

3. shortening the available run time of a system's battery.

By way of comparative example, the BLU (18) in a 12.1″ XGA displaymodule is designed to achieve a maximum continuous screen brightnessB(0) of 500 nits (i.e., cd/m²). The LED strip of the LED array (21) insuch a BLU (18) is designed to dissipate approximately 5 W continuouslyto produce the preceding continuous screen brightness B(0). Inaccordance with the present disclosure, the LED array (21) is replacedby the LED array (05) that is capable of emitting substantially morelight without exceeding the maximum allowable current for its LEDs (01).For example, if the LED array (05) were to safely operate atapproximately 20 W, the resulting boosted screen brightness B(nos) couldbe approximately 3000 nits. Lacking the high LED density LED array (05),it is unlikely that a conventional LED array (21) could withstanddissipating such a power increase without failing or melting somethinginside the tablet (06). However, a conventional consumer gradeelectronic device might survive without damage a more modest temporary1.25 to 2.0 brightness increase, i.e 625 nits to 1000 nits.

However, such increased power dissipation could not be sustainedindefinitely without modifying cooling of the LED array (21). A BLU (18)in accordance with the present disclosure allows the LCD module (08) tooperate for at least 15 seconds without exceeding the BLU's temperatureor current limits. The combination of the DPS (13), the BLU brightnesscontrol (22) and the LED driver (09) permits a user to initiate periodsof increased screen brightness B(nos) either using the NOS button (07)or via a soft button control or other convenient means for temporarilyincreasing electrical power supplied to the LEDs (01) as much as 3 timesor more for a period of 2-20 seconds or more before returning to itspreselected continuous brightness setting.

Consider the BLU (18) in a conventional 12.1″ XGA display module beingdriven at its normal, continuous maximum brightness, e.g. 5 W, and theequilibrium temperature directly adjacent to the LED array (21) is 35°C. Also, assume that the maximum manufacturer-rated operatingtemperature for this display module is 70° C. Further assume thatincreased screen brightness B(nos) were enabled for a preselectedduration of 15 seconds. Hypothetically, based upon operational testingwhile designing the conventional 12.1″ XGA display module it were knownthat a 15 second 15 W input power increase to the LED array (21) causesa temperature increase of 15° C. Since a temperature of 50° C. is stillwell within the normal operating parameters both for the LCD glass panel(04) and for the BLU (18), then the DPS (13) would permit such anincrease in screen brightness to B(nos).

After such a 15 second period of increased screen brightness B(nos) whenthe BLU (18) returns to its original power and brightness level, assumethat a user were to immediately start another period of increased screenbrightness B(nos). Since insufficient time has elapsed for heating dueto the increased screen brightness B(nos) to have fully dissipated, theinitial starting temperature of the LCD glass panel (04) and of the BLU(18) for the subsequent 15 second period of increased screen brightnessB(nos) is approximately 50° C. After a second 15 seconds of 15 W powerinput to the LED array (21) its temperature would be approximately 65°C., (actually somewhat less but this is not important to the currentexample). Again, this is within the normal operating range of thisdisplay module so the DPS (13) would permit such a second increase inscreen brightness to B(nos).

Again, immediately after the second increase in screen brightness B(nos)a user were to immediately start a third period of increased screenbrightness B(nos). The projected temperature of the LCD glass panel (04)and of the BLU (18) at the end of the period will be somewhat less than80° C. which exceeds the temperature rating for the conventional 12.1″XGA display module. Therefore, the DPS (13) would not permit the thirdsuccessive period of increased screen brightness B(nos) until thetemperature of the LCD glass panel (04) and of the BLU (18) hasdecreased to approximately 55° C. at which time another period ofincreased screen brightness B(nos) could be safely permitted.

As is readily apparent, including the thermal sensors (16) in the LCDmodule (08) assists in preventing the LEDs (01) from being damaged by aperiod of increased screen brightness B(nos). Alternatively, if the DPS(13) records how much the power has been increased to the LEDs (01) andfor what duration and how much time has elapsed since the most recentperiod of increased screen brightness B(nos), it is also possible toprevent the LEDs (01) from being damaged by a period of increased screenbrightness B(nos).

FIGS. 11 and 12 illustrate perhaps the simplest structure for a highlyeffective thermal ballast (10). FIGS. 11 and 12 depict a preferredembodiment of FIG. 8's thermal ballast (10) that includes:

-   -   1. a highly thermally conductive planar layer (42), typically        0.1 to 3.0 mm thick, made from, for example, graphite, aluminum        or copper on one side of which is laminated;    -   2. a relatively thin thermal storage layer (44), for example 0.5        to 10 mm thick, that is preferably filled with a PCM (46).        While the thermal storage layer (44) is preferably filled with        the PCM (46), a still useful but less effective thermal ballast        (10) can be assembled using a thermal storage layer (44) made        solely from a solid piece of material advantageously having a        high specific heat, e.g. aluminum.

Within the assembled tablet (06), juxtaposing one end of the highlythermally conductive layer (42) with the LED array (05, 21) establishesa thermal connection there between. Extending one end of the highlythermally conductive layer (42) out beyond an edge of the thermalstorage layer (44) can facilitate this thermal coupling between thehighly thermally conductive layer (42) and the LED array (05, 21).

Since many PCMs exhibit comparatively poor thermal conductivity, using agraphite highly thermally conductive layer (42) is highly advantageous.For example, the literature value for the thermal conductivity ofGlauber's salt is approximately 0.6 W/(m·K) whereas the thermalconductivity of many commercially available graphite sheet materials is400 W/(m·K) or more. Thus, the planarized thermal ballast (10) thatincludes PCM(s) (46), such as that depicted in FIGS. 11 and 12, bothefficiently spreads heat away from the LED array (05, 21) via thegraphite highly thermally conductive layer (42) while concurrentlyproviding substantial thermal energy storage in the thermal storagelayer or layers (44).

If the highly thermally conductive planar layer (42) is made fromgraphite it may include a thin (e.g. approximately 0.01 to 0.03 mmthick), laminated polymeric isolation layer on one or both sides. Tofacilitate assembling the layered structure of thermal ballast (10)depicted in FIGS. 11 and 12, a thin adhesive coating may be appliedeither to the highly thermally conductive planar layer (42) or to thethermal storage layer (44) for bonding them together into an integralunit.

As would be apparent to those skilled in the art, there exist manypossible alternative specific lamellar structures that include highlythermally conductive planar layer or layers (42) and thermal storagelayer or layers (44) that implement the general concept of thermalballast (10) depicted in FIGS. 11 and 12. For example, the thermalstorage layer or layers (44) can be formed by a plate of porous metalinto which pores PCM (46) has been infused. One example of a suitableporous metal for forming part of the thermal storage layer or layers(44) is porous aluminum marketed by Exxentis Ltd. located inSwitzerland. Exxentis Ltd.'s products include seven (7) different typesof porous aluminum material each having a different pore size. The poresize selected for porous aluminum included in the thermal storage layer(44) must be smaller than the thickness thereof. Note that other porousmaterials such as a porous stainless steel, bronze, nickel, nickel basedalloy, titanium or copper material could be used for the thermal storagelayer (44) instead of porous aluminum, If a porous metal plate were toalso include an integrally formed layer of solid material that is 1.0 mmto 10.0 mm thick, the solid layer could advantageously function as thehighly thermally conductive planar layer (42) thereby providing aone-piece thermal ballast (10).

Advantageously, as illustrated in FIG. 13 the PCM (46) can be packagedin a thin inert casing or pouch (48) such as is commonly used forlithium ion batteries. For example, pouches (48) having a wallthicknesses of 71 to 156 μm are commercially available, and can beordered to have a depth perpendicular to the highly thermally conductivelayer (42) of 4.5 mm to 8.0 mm. Multi-layer pouches (48) include a nylonlayer, an AL foil layer, and a cast polypropylene layer. Using theflexible pouch (48) for the thermal storage layer (44):

-   -   1. provides a good vapor barrier;    -   2. exhibits excellent pouch sealing;    -   3. provides good chemical resistance; and    -   4. can stretch and give thereby accommodating thermal expansion        and contraction of the PCM (46).        Consequently, even though the PCM (46) changes from a solid to a        liquid when heated above its transition temperature, the sealed        pouch (48) will hold the PCM (46) in place without leakage.

The PCM (46) enclosed within the thermal storage layer (44) might, forexample, be Glauber's salt, the decahydrate of sodium sulfateNa₂SO₄:10H₂O which is also identified by the names sal mirabilis(decahydrate), mirabilite (decahydrate) and disodium sulfatedecahydrate. For use in the thermal storage layer (44), Glauber's saltadvantageously undergoes a phase change at approximately 90° F., i.e.well within the 85° C. maximum operating temperature of LEDs (01)included in the LED arrays (05, 21). In addition to Glauber's salt,other phase change materials, both organic and inorganic, are known eachwith its unique phase transition temperature. For example an organicmixture of materials known as OM65P, made by RGEES, LLC, 1465 Sand HillRoad, Candler, N.C., changes phase at 149° F., also well within the 85°C. maximum operating temperature of LEDs (01).

Filling the thermal storage layer (44) with a PCM (46) significantlyincreases the heat storage capacity of the planar thermal ballast (10)if the temperature rises above the PCM's transition temperature.Strictly by way of example to illustrate the relative increase inthermal storage capacity possible by using a PCM (46), if the thermalstorage material were ice (an unlikely choice), below its freezing pointthis material would absorb about 0.5 cal/g/° C. Therefore, for every 0.5calorie of heat absorbed by one gram of ice its temperature increases byone degree Celsius. However, at ice's melting point of 0° C., the iceabsorbs about 80 cal/g/° C., a 160× increase in heat absorption capacityin comparison with solid ice.

As would be apparent to those skilled in the art, a planarized thermalballast (10) having multiple thermal storage layers (44), that in oneembodiment respectively contact opposite sides of the highly thermallyconductive layer (42), can be fabricated with each individual thermalstorage layer (44) enclosing a PCM (46) having a different transitiontemperature. Such a multi-layered planarized thermal ballast (10)extends the temperature range over which the a planarized thermalballast (10) provides enhanced thermal energy storage capacity for theLEDs (01) included in the LED array (05, 21). Also by way of example,the thermal ballast (10) could include several highly thermallyconductive layers (42) each of which is laminated on one or both sidesto thermal storage layers (44). For practical reasons assuming that thePCMs (46) are not otherwise microencapsulated, it may be advantageous toenclose each PCM (46) having a different transition temperature withinan individual thermal storage layer (44) rather than mixing the PCMs(46) together inside a single thermal storage layer (44).

Thus far, only the heat storage aspect of the planarized thermal ballast(10) has been discussed. However, in practical use, heat accumulated inthe PCM must ultimately be dissipated into the ambient environment.Also, from the perspective of a product designer it would beadvantageous to integrate the thermal ballast (10) into the heatexchanger, i.e. a heat sink (50) having fins (52). Such heat exchangersare widely used in many electronic products to dissipate the heatproduced during the product's operation. FIG. 14 and FIG. 15 illustrateone possible example in which the thermal ballast (10) is fullyintegrated into a heat sink (50) having fins (52). In an alternativeembodiment not illustrated in the drawing FIGs the fins (52) could behollow and filled with PCM (46).

An advantage of the integrated thermal design shown in FIGS. 14 and 15is that when the system power consumption establishes an equilibriumtemperature within normal parameters, the PCM (46) within the thermalstorage layer (44) remains in its solid phase and has little or noinfluence on the operating temperature of relevant power components.However, if the product's power consumption increases such that itexceeds the heat dissipation capability of the heat sink (50), excessheat will be stored in the PCM (46) present within the thermal storagelayer (44) thereby forestalling the product from exceeding its maximumoperating temperature for some interval of time.

Various design alternatives for the heat sink (50) are possible. Forexample, since all practical PCMs (46) currently identified have arelatively low thermal conductivity, creating a cavity in the base of afinned heat sink (50) and filling the cavity with a PCM (46) couldimpede the heat conduction from source to sink (i.e. the fins (52)).Secondly, as demonstrated by the thermal modeling of the planarizedthermal ballast (10) described in greater detail below, the effectiveperformance of the PCM (46) is strongly affected by how well and howintimately the highly thermal conductive layer (42) can transfer heatinto the PCM (46). One way in which the preceding issues can beameliorated illustrated in FIG. 16 is to include an array of thermalvias (54) from the heat sink base through the PCM containing thermalstorage layer (44). Here, the term “thermal via” is appropriated fromprinted circuit board (PCB) design where multiple small holes aredrilled into a PCB, particularly around power components, which holesare then subsequently electroplated with metal to allow heat transferfrom one layer of the PCB to other layers thereof. However, in theinstance of the thermal ballast heat sink (50) depicted in FIG. 16, thethermal vias (54) are more likely to be small solid metal posts.

As would be apparent to those skilled in the art, there exist manypractical uses for the thermal ballast (10) and the heat sink (50) asdescribed herein other than for use in thermal protection of LEDs (01)included in the LED array (05, 21). Any product or system, for example,a battery powered data processor or circuit board, that needs to extendits operating time during adverse thermal operating conditions couldbenefit from use of a PCM-based, thermal ballast as described herein. Itis anticipated that such embodiments could be relatively small and wouldmount on, over or adjacent to such critical heat-generating components.

While a PCM-based thermal ballast (10) can significantly increase theamount of heat that can be temporarily absorbed from a heat source,ultimately, the thermal ballast (10) has a finite heat absorbingcapacity. Therefore, and as discussed in detail above, part of thethermal design of this or any heat absorbing system must include a meansby which the PCM can externally dissipate absorbed heat. In the instanceof the BLU (18) systems described herein, typically the DPS (13)monitors the overall system thermal environment and prevents the systemfrom inadvertent thermal overload. The primary means for avoidingthermal overload is by allowing sufficient time for the system todissipate excess heat into the ambient environment. This scenario willbe subsequently discussed in greater detail.

It should be noted that, due to thermal expansion of PCM (46), leakagethereof might possibly occur with a more ridged containment vessel suchas the thermal storage layer (44) illustrated in FIGS. 11 and 12 thanwith the pouch (48) illustrated in FIG. 13. This leakage possibility,which can be avoided in several different ways, arises if the PCM (46)when heated expands more than the concurrent expansion of the thermalstorage layer (44). One technique for avoiding leakage is if duringfilling and subsequent sealing of the thermal storage layer (44) the PCM(46) is maintained at or above the maximum operating temperature of thethermal ballast (10). Presuming that the PCM (46) expands when heated,in this way the volume occupied by the PCM (46) will never exceed thecapacity of the thermal storage layer (44) thereby avoiding any buildupof a positive pressure within the thermal storage layer (44).Alternatively, the thermal storage layer (44) can be slightlyunderfilled to thereby leave a small amount of air or other gas withinthe thermal storage layer (44) after sealing. The operation of thethermal ballast (10) will be optimized if such a bubble air or other gaswithin the thermal storage layer (44) is just large enough toaccommodate thermal expansion of the PCM (46). Other practical ways foravoiding leakage of the PCM (46) due to thermal expansion that are notdescribed above might be developed for fabricating a thermal storagelayer (44) that operates in accordance with the present disclosure.

Alternatively, if the thermal storage layer (44) includes a plate ofporous metal the plate is immersed in a bath of PCM (46) that has beenheated to above its transition temperature. If a chemical barrier layeris needed between the porous metal and the PCM (46), before immersionthe porous metal plate is first filled with a chemical barrier materialsuch as a polymeric material perhaps silicone that has been diluted in asolvent which solvent is then evaporated out leaving only a thin coatingof the polymeric material on the walls of the porous metal plate's opencells. While maintaining the PCM (46) above its transition temperature,the bath including the immersed porous metal plate is placed in a vacuumchamber that is then evacuated to remove air. The vacuum level needs tobe only in a range of 0.1 mm to 5.0 mm of mercury although, dependingupon vacuum pressure's efficacy, a lower or higher vacuum than thisrange may be useful. After reaching the desired vacuum level, thechamber is backfilled thereby forcing the PCM into the open pores of themetal plate. After the porous metal plate now infused with the PCM (46)has returned to atmospheric pressure and allowed to cool to below thetransition temperature of the PCM (46), coating the PCM filled platewith a polymeric sealing material such as a liquid silicone which isthen either air and/or catalytically cured completes fabrication of thethermal storage layer (44).

To illustrate the operating time extension that is readily obtained byemploying the PCM based, planarized thermal ballast (10) as describedherein, a specific example is evaluated. The LED backlight in a batteryoperated, 10.1 inch diagonal, WXGA (i.e., 1280×800 resolution) LCDmodule (08) nominally consumes 4 watts (4 W) of electrical power. FIG.11 and FIG. 12 show the mechanical design of the PCM-based, planarizedthermal ballast (10) used as the basis for thermal modeling. The LEDs(01) in the 10.1 inch diagonal, WXGA's backlight are able to operatewithin their normal operating temperature limits as long as batterypower does not exceed 4 watts. The thermal model of the 10.1 inchdiagonal, WXGA's backlight calculates the temperature of the LEDs (01)of the LED array (05, 21) of the BLU (18) over time at four power inputlevels: 4 W, 8 W, 12 W and 16 W. The LED array (05, 21) includes a215×4.5×1.5 mm aluminum strip with a printed circuit on the uppersurface thereof (i.e., facing away from the highly thermally conductivegraphite layer (42). The LED array (21) is thermally coupled to the longedge of a 1.0 mm thick highly thermally conductive graphite layer (42)having an in-plane thermal conductivity of 400 W/(m·K), and is laminatedto a 215×115×4.5 mm PCM filled thermal storage layer (44) depicted inFIGS. 11 and 12. The PCM (46) filling the thermal storage layer (44)changes phase at 65° C. The maximum allowable operating temperature forthe LEDs (01) is 85° C.

Heat from the LEDs (01) enters the BLU (18) at the LED array (05, 21).Heat exits the portable tablet (06) primarily from the outer surface ofthe thermal storage layer (44) (i.e., its bottom surface) at a rate of 5W/(m²/K). For comparison, the temperature of the LEDs (01) versus timeat 4 W electrical power input to the LED array (05, 21) is appears inall three graphs (i.e. in FIGS. 17, 18 and 19). Note that at 4 W inputto the LED array (05, 21), the operating temperature of the LEDs (01)reaches a maximum approximately 64° C. which does not cause the PCM (46)to change phase.

FIG. 17 shows the operating temperature of the LEDs (01) versusoperating time with 8 W of input power to the LED array (05, 21). CurveG1-1 shows that in the absence of a PCM phase change the LEDs (01):

-   -   1. reach their maximum operating temperature i.e. 85° C., in        approximately 60 minutes; and    -   2. afterwards the LEDs (01) would be operating above their        maximum allowed 85° C. operating temperature.        Curve G1-2 shows the operating temperature rise of the LEDs (01)        for the graphite thermal ballast (10) when the PCM (46) within        the thermal storage layer (44) changes phase. The operating        temperature of the LEDs (01) remains under 85° C. for nearly 160        minutes, more than doubling the safe operating time in        comparison with when a phase change does not occur as depicted        in curve G1-1. By way of comparison, curve G1-3 shows the result        of replacing the thermal storage layer (44) filled with PCM with        an aluminum thermal ballast (10) of the same dimensions. Curve        G1-4 demonstrates that using a multilayered PCM design for the        thermal ballast (10) further slows the operating temperature        increase of the LEDs (01). Note that any of the planarized        thermal ballast (10) that includes PCM (46), depicted in the        curves G1-2 and G1-4, nearly doubles the safe operating time in        comparison with using an aluminum thermal ballast (10) of the        same dimensions shown by the curve G1-3.

FIG. 18 shows the operating temperature of the LEDs (01) versusoperating time with 12 W of input power to the LED array (05, 21). CurveG2-1 shows that in the absence of a PCM phase change the LEDs (01) reachtheir maximum operating temperature i.e. 85° C., in approximately 29minutes. Curve G2-2 shows the result when the PCM (46) within thethermal storage layer (44) changes phase. The operating temperature ofthe LEDs (01) remains under 85° C. for approximately 60 minutes, morethan doubling the operating time in comparison with when a phase changedoes not occur as depicted in curve G2-1.

Curve G2-3 shows that juxtaposing opposite sides of the highly thermallyconductive layer (42) respectively with half thickness thermal storagelayers (44) improves temperature control of the LEDs (01) during phasetransition of the PCM (46).

Curve G2-4 shows the beneficial effect on the operating temperature ofthe LEDs (01) that results from:

-   -   1. doubling the overall heat conduction in the highly thermally        conductive layer (42); and    -   2. juxtaposing opposite sides of the highly thermally conductive        layer (42) respectively with half thickness thermal storage        layers (44).        As illustrated in curve G2-4, the safe operating time for the        LEDs (01) is increased by over 2.5× in comparison with an        absence of a PCM phase change (i.e., curve G2-1).

Curves G2-2, G2-3 and G2-4 in FIG. 18 show that with substantiallyincreased input power to the LEDs (01), the specific design and materialselections in the planarized thermal ballast (10) can significantlyreduce the rate of temperature increase in the LEDs (01) of the LEDarray (05, 21) when the LEDs (01) are supplied with a large amount ofelectrical power.

Finally, FIG. 19 shows the temperature of the LEDs (01) versus operatingtime with 16 W of input power to the LED array (05, 21). Curve G3-1shows that, in the absence of a PCM phase change, the LEDs (01) reachtheir maximum operating temperature, i.e. 85° C., under the modeledconditions in approximately 18 minutes. Curve G3-2 shows the result whenthe PCM (46) within the thermal storage layer (44) changes phase. Theoperating temperature of the LEDs stays under 85° C. for approximately30 minutes.

Again, curves G3-2 and G3-3 illustrate how the performance of theplanarized thermal ballast (10) can be improved by its specific designdetails. Particularly, the temperature rise of the LEDs (01) during thePCM phase transition can be flattened by improved heat flow into the PCM(46) within the thermal storage layer (44) thereby extending the systemoperating time.

Although the present disclosure has been described in terms of thepresently preferred embodiment, it is to be understood that suchdisclosure is purely illustrative and is not to be interpreted aslimiting. For a portable video monitor, the screen brightness can betemporarily increased almost instantaneously by as much as 2-6 timesabove its continuous operation brightness. However, a screen brightnessincrease of as little as 1.25× would still be within the scope of thepresent disclosure. Also within the scope of the present disclosurewould be simply overdriving LEDs (01) of a conventional LED array (05),although as explained previously such operation carries a number ofdisadvantages. Consequently, without departing from the spirit and scopeof the disclosure, various alterations, modifications, and/oralternative applications will, no doubt, be suggested to those skilledin the art after having read the preceding disclosure. Accordingly, itis intended that the following claims be interpreted as encompassing allalterations, modifications, or alternative applications as fall withinthe true spirit and scope of the disclosure including equivalentsthereof. In effecting the preceding intent, the following claims shall:

1. not invoke paragraph 6 of 35 U.S.C. § 112 as it exists on the date offiling hereof unless the phrase “means for” appears expressly in theclaim's text;

2. omit all elements, steps, or functions not expressly appearingtherein unless the element, step or function is expressly described as“essential” or “critical;”

3. not be limited by any other aspect of the present disclosure whichdoes not appear explicitly in the claim's text unless the element, stepor function is expressly described as “essential” or “critical;” and

4. when including the transition word “comprises” or “comprising” or anyvariation thereof, encompass a non-exclusive inclusion, such that aclaim which encompasses a process, method, article, or apparatus thatcomprises a list of steps or elements includes not only those steps orelements but may include other steps or elements not expressly orinherently included in the claim's text.

The invention claimed is:
 1. A thermal ballast (10) adapted fortemporarily storing heat generated during a heat generating device'soperation thereby extending the device's operating time during aninterval of adverse thermal operating condition, the thermal ballast(10) being comprised of: a. a highly thermally conductive layer (42)that is adapted for being thermally coupled to the heat generatingdevice; and b. a thermal storage layer (44) that includes a porousmaterial, and that is laminated to the highly thermally conductive layer(42).
 2. The thermal ballast (10) of claim 1 wherein the highlythermally conductive layer (42) is a sheet of graphite.
 3. The thermalballast (10) of claim 1 wherein the highly thermally conductive layer(42) is a sheet of copper.
 4. The thermal ballast (10) of claim 1wherein the thermal storage layer (44) includes a material (46) thatchanges phase at a temperature below the heat generating device'smaximum operating temperature.
 5. The thermal ballast (10) of claim 4wherein the material (46) that changes phase is microencapsulated. 6.The thermal ballast (10) of claim 4 wherein inner surfaces of thethermal storage layer (44) include a thin chemical barrier layer toprevent chemical reaction with the material (46) that changes phase at atemperature below the heat generating device's maximum operatingtemperature.
 7. The thermal ballast (10) of claim 4 wherein the thermalstorage layer (44) and the highly thermally conductive layer (42) areinterleaved into a stack having multiple layers.
 8. The thermal ballast(10) of claim 1 wherein the thermal storage layer (44) includes acombination of materials (46) that respectively change phase atdiffering temperatures which respectively are below the heat generatingdevice's maximum operating temperature.
 9. The thermal ballast (10) ofclaim 8 wherein the phase change materials (46) are individuallymicroencapsulated and then mixed to form a homogeneous material whichcollectively undergoes phase changes at differing temperatures.
 10. Therthermal ballast (10) of claim 8 wherein the porous material infused withthe phase change material (46) that forms the thermal storage layer (44)further includes an integrally formed layer of solid material thatprovides the highly thermally conductive layer (42).
 11. The thermalballast (10) of claim 1 having at least two thermal storage layers (44),the highly thermally conductive layer (42) being laminated to one of thethermal storage layers (44) on opposite sides respectively of the highlythermally conductive layer (42).
 12. The thermal ballast (10) of claim 1wherein the highly thermally conductive layer (42) is aluminum.
 13. Thethermal ballast (10) of claim 12 wherein inner surfaces of the thermalstorage layer (44) include a thin chemical barrier layer to preventchemical reaction with a material (46) that changes phase at atemperature below the heat generating device's maximum operatingtemperature that is included in the thermal storage layer (44).
 14. Thethermal ballast (10) of claim 12 wherein thermal storage layers (44)includes a material (46): a. that changes phase at a temperature belowthe heat generating device's maximum operating temperature; and b. thatis microencapsulated.
 15. The thermal ballast (10) of claim 1 whereinthe highly thermally conductive layer (42) includes a hollow heat sink(50): a. that also provides the thermal storage layer (44) of thethermal ballast (10); and b. the hollow heat sink (50) being filled witha material (46) that changes phase at a temperature below the heatgenerating device's maximum operating temperature.
 16. The thermalballast (10) of claim 1 wherein the porous material included in thethermal storage layer (44) is porous aluminum.
 17. The thermal ballast(10) of claim 1 wherein pores within the porous material included in thethermal storage layer (44) are coated with a chemical barrier layerbefore the porous material is infused with the phase change material(46).
 18. The thermal ballast (10) of claim 4 wherein the porousmaterial infused with the phase change material (46) that forms thethermal storage layer (44) further includes an integrally formed layerof solid material that provides the highly thermally conductive layer(42).